The present invention relates to a process for producing acicular magnetic iron oxide particles for magnetic recording which have a high coercive force and an excellent erasing characteristic.
With progressing miniaturization and weight-reduction of magnetic recording and reproducing apparatuses in recent years, the necessity for a recording medium having a higher performance such as a magnetic tape and a magnetic disk has been increasing more and more. In other words, a magnetic recording medium is required to have a higher recording density.
In order to improve the recording density of a magnetic recording medium, the particles of the magnetic material of the magnetic recording medium must have as high a coercive force as possible. This fact is described in, for example, Reports of Technical Researches of the Institute of Electronics and Communication Engineers of Japan, MR77-36 (1987), p. 37, "In order to improve the recording density of a magnetic tape, it is necessary to enhance the coercive force of the magnetic powder which is used for the tape".
As acicular magnetic iron oxide particles having a high coercive force, (1) Co-doped acicular magnetic iron oxide particles and (2) Co-coated acicular magnetic iron oxide particles are conventionally known. The coercive force of these acicular iron oxide particles have a tendency of enhancing with the increase of the Co content. The Co-doped particles (1) are obtained by adding a Co salt to the formulation reaction system of the acicular goethite particles before the reaction, thereby obtaining Co-containing acicular goethite particles, and reducing the Co-containing acicular goethite particles to obtain Co-doped acicular magnetite particles or further oxidizing the thus-obtained magnetite particles to obtain Co-doped acicular maghemite particles. The Co-coated particles (2) are obtained by reducing the acicular goethite particles to obtain acicular magnetite particles or further oxidizing the thus-obtained acicular magnetite particles to obtain acicular maghemite particles, and modifying the surface of the acicular magnetite particles or acicular maghemite particles as precursor particles, thereby obtaining acicular magnetite particles or acicular maghemite particles coated with Co or Co and Fe(II).
Since a magnetic recording medium is repeatedly used for a long time for recording and erasing, it is required to have an excellent erasing characteristic.
In order to satisfy this demand against a magnetic recording medium, it is necessary that the magnetic iron oxide particles used for the magnetic recording medium have an excellent erasing characteristic.
Japanese Patent Publication Nos. 60-51242 (1985) and 60-51243 (1985) and Japanese Patent Application Laid-Open (KOKAI) No. 61-111508, for example, describe a method of improving various properties of Co-coated acicular magnetic iron oxide particles by adding an Sr salt or a Ba salt together with a Co salt at coating the magnetic iron oxide particle with Co. Japanese Patent Application Laid-Open (KOKAI) Nos. 55-72007 (1980), 59-50504 (1984), 59-159502 (1984), 60-165703 (1985), 1-219025 (1989) and 1-290534 (1989), for example, describe a method of improving various properties of Co-coated acicular magnetic iron oxide particles by adding at least one selected from the group consisting of Si compound, Al compound, Zn compound, Mg compound and Ca compound together with a Co salt at coating the magnetic iron oxide particle with Co.
As described above, acicular magnetic iron oxide particles having a high coercive force and an excellent erasing characteristic are in the strongest demand at present. Though the above-described Co-doped acicular magnetic iron oxide particles have a high coercive force, the coercive force distribution enlarges due to the diffusion of Co in crystals, etc., resulting in the deterioration in the erasing characteristic.
This phenomenon is described in the said Reports of Technical Researches of the Institute of Electronics and Communication Engineers of Japan, "Co-solid solution (doped) magnetic iron oxide powder suffers from a serious defect in that since the coercive force thereof is apt to change with heat and passage of time, when a tape is produced by using the magnetic powder, the thus-obtained tape is inferior in print through and erasing characteristic. This defect is considered to be caused by the fact that Co ions move in crystals even at room temperature."
In contrast, the said Co-coated acicular magnetic iron oxide particles have not only a high coercive force but also a superior erasing characteristic as compared to that of Co-doped magnetic iron oxide particles. This phenomenon is described in the said Reports of Technical Researches of the Institute of Electronics and Communication Engineers of Japan, " . . . In the Co-epitaxial (Co-coated) magnetic iron oxide powder, since it has a double structure, these defects are solved. The Co-epitaxial magnetic iron powder is stable thermally independance to change with time, and the tape produced by using the magnetic powder has excellent print through and erasing characteristic . . . "
However, there is no end to the demand for the improvement of the erasing characteristic, and it is pointed out that the said Co-coated acicular magnetic iron oxide particles cannot be said to have a satisfactory erasing characteristic, since the coercive force distribution thereof still enlarges.
This fact is described in, for example, Japanese Patent Application Laid-Open (KOKAI) No. 61-17426 (1986), " . . . It has been found that in the above-described magnetic powder using the .gamma.-Fe.sub.2 O.sub.3 particles, the coercive force distribution has a tendency of enlarging as the .gamma.-Fe.sub.2 O.sub.3 particles become smaller, and a tendency of further enlarging if these particles are coated with cobalt. . . . If the above-described Co-coated .gamma.-Fe.sub.2 O.sub.3 particles are made smaller in order to realize high-density recording, although a predetermined coercive force (Hc) is obtained, the magnetic powder is deteriorated in the coercive force distribution and is inferior in the erasing characteristic. . . . "
The coercive force distribution of Co-coated acicular magnetic iron oxide particles has a tendency of enlarging with the increase of the Co content, resulting in the deterioration in the erasing characteristic. That is, there is a reverse correlation between the improvement of the coercive force distribution and the improvement of the coercive force.
Accordingly, it is a technical problem of the present invention to improve the erasing characteristic while maintaining the coercive force of Co-coated acicular magnetic iron oxide particles.
As a result of the present inventors' ernest study, it has been found that in a process for producing acicular magnetic iron oxide particles comprising heat-treating at 50.degree. to 100.degree. C., an alkaline suspension solution which is obtained by mixing an aqueous dispersion of acicular magnetic iron oxide particles, an aqueous solution of a Co salt and an alkaline aqueous solution or by mixing an aqueous dispersion of acicular magnetic iron oxide particles, an aqueous solution of a Co salt, an aqueous solution of an Fe(II) salt and an alkaline aqueous solution so as to coat (modify) the acicular magnetic iron oxide particles with Co or Co and Fe(II), by adding an Sr salt and/or a Ba salt to one selected from the group consisting of the aqueous dispersion of acicular magnetic iron oxide particles, the aqueous solution of a Co salt, the aqueous solution of an Fe(II) salt, the alkaline aqueous solution and the alkaline suspension solution before heat-treatment, and adding at least one selected from the group consisting of Si compound, Al compound, Zn compound and Mg compound to one selected from the group consisting of the aqueous dispersion of acicular magnetic iron oxide particles, the aqueous solution of a Co salt, the aqueous solution of an Fe(II) salt, the alkaline aqueous solution, the alkaline suspension solution before heat-treatment and the alkaline suspension solution during heat-treatment, the said technical problems have been solved. The present invention has been attained on the basis of this finding.